The second volume can be found here:

https://books.google.com/books?id=XyoAAAAAQAAJ&printsec=frontcover&dq=TRAIT%C3%89+%C3%89L%C3%89MENTAIRE+D'ART+MILITAIRE+ET+DE+FORTIFICATION&hl=en&sa=X&ved=0ahUKEwiE-e-ExtLiAhXwT98KHcCrBC4Q6AEILTAA#v=onepage&q=TRAIT%C3%89%20%C3%89L%C3%89MENTAIRE%20D'ART%20MILITAIRE%20ET%20DE%20FORTIFICATION&f=false

The first volume is here:

https://books.google.com/books?id=IjQVAAAAQAAJ&printsec=frontcover&dq=TRAIT%C3%89+%C3%89L%C3%89MENTAIRE+D'ART+MILITAIRE+ET+DE+FORTIFICATION&hl=en&sa=X&ved=0ahUKEwiE-e-ExtLiAhXwT98KHcCrBC4Q6AEINTAB#v=onepage&q=TRAIT%C3%89%20%C3%89L%C3%89MENTAIRE%20D'ART%20MILITAIRE%20ET%20DE%20FORTIFICATION&f=false

And it's on Amazon here:

Volume 1:

https://www.amazon.com/Élémentaire-Militaire-Fortification-Polythechnique-Militaires/dp/1173577343/ref=sr_1_1?keywords=Trait%C3%A9+%C3%A9l%C3%A9mentaire+d%27art+militaire+et+fortification%2C+%C3%A0+l%27usage+des+%C3%A9l%C3%A8ves+l%27%C3%A9cole+polythechnique+et+des+%C3%A9l%C3%A8ves+des+%C3%A9coles+militaires&qid=1559744880&s=books&sr=1-1

Volume 2:

https://www.amazon.com/Élémentaire-Militaire-Fortification-Polytechnique-Litterature/dp/201372084X/ref=sr_1_fkmr0_1?keywords=Trait%C3%A9+%C3%A9l%C3%A9mentaire+d%27art+militaire+et+fortification%2C+%C3%A0+l%27usage+des+%C3%A9l%C3%A8ves+l%27%C3%A9cole+polythechnique+et+des+%C3%A9l%C3%A8ves+des+%C3%A9coles+militaires&qid=1559744950&s=books&sr=1-1-fkmr0

I hope this helps.